T cell-derived soluble factors which regulate antibody synthesis will be studied. The molecular weights of purified murine and human enhancing factors (EF) will be confirmed by analytical techniques such as SDS-polyacrylamide gel electrophoresis and membrane ultrafiltration, and their modes of action in affecting the immune response of purified B lymphocytes studied by examining DNA synthesis, EF- and antigen-binding and distribution of immunoglobulin surface receptors. Suppressor molecules from specifically-stimulated mouse spleen cells will be partially purified and their molecular weights, physiocochemical properties, cellular source and possible modes of action determined. Studies will be initiated to determine whether T cell-derived mediators can affect the function of T lymphocytes involved in cell-mediated immunity. BIBLIOGRAPHIC REFERENCES: Cerny, J., E.B. Waner, and A.S. Rubin. 1975. T cell products activating stem cells. Further studies on the origin and action of the factor(s). J. Immunol. 115:513. Rubin, A.S., and A.B. MacDonald. 1976. Partial characterization of an immuno-enhancing factor from allogeneic human lymphocyte cell lines. Immunology 30:69.